1. Field of the Invention
The present invention relates to an improved data transmission system in which asynchronous data can be transmitted over long distances at rates heretofore used for short distance asynchronous or long distance synchronous transmission.
2. Description of the Prior Art
Asynchronous to synchronous data conversion circuitry of the prior art is generally limited either by the asynchronous data rates, the synchronous data rates, particularly character formats, or by the capability in the number of asynchronous characters which can be transmitted end-to-end continuously. Furthermore, these asynchronous-to-synchronous converters of the prior art are external devices requiring additional data cabling and in many instances, power cabling.
Asnychronous data transmission of the prior art has generally been limited to data rates of 1300 bits per second or less over voice grade telecommunication transmission lines. At present, data terminals have been designed to operate asynchronously at higher data rates i.e., at 2400 bits per second, 4800 bits per second and 9600 bits per second; which rates were previously used only for synchronous data transmission.
A problem associated with the aforementioned high asynchronous data rates is loss of synchronization and errors in data transmission when such high speed asynchronous data terminals are located remotely from the data processors with which they must communicate. This has resulted in the prior art in the use of data rates of 1800 bits per second or less for high speed asynchronous data transmission whenever such transmission is required generally over distances greater than several hundred feet.
Asynchronous-to-synchronous converters are well known and are commonly used for data transmission rates for 2400 bits per second or higher. Such prior art converters interface asynchronous data terminals to synchronous modems or other equipment which operates at the same or higher data rates. In such converters, characters are recognized based upon the start bits in the asynchronous data transmission, the characters are temporarily stored in a memory and are serially transmitted together with start/stop bits to a synchronous transmission modem using the synchronous clocks from the modem.
Where it is desired to perform a data rate conversion between a higher synchronous data rate and a lower asynchronous data rate, the steps described in the immediately preceeding paragraph are performed in reverse sequence at the receiving modem. When a data rate conversion is not required, the received synchronous transmission from the modem can be transmitted directly to an asynchronous data terminal.